1. Field of the Invention
The present invention relates to soldering, and more particularly, to an improved method and apparatus for making soldered joints for assemblages of electronic components.
2. Description of the Prior Art
Methods of making soldered joints in devices where many electronic components are electrically joined to an interconnecting pattern on a common base of insulated material, such as printed wiring boards (PWB), may be classified as either discrete joint soldering, where each joint is soldered individually by a soldering iron, or batch soldering such as wave soldering, where the PWB with its full complement of components in position, is subjected to a molten wave of solder. Heretofore, because of its relative speed, wave soldering was widely utilized, particularly where a large number of soldered joints were required on a single PWB. However, there are certain disadvantages in wave soldering, for example, the individual components are usually mechanically fastened to the PWB in some manner, prior to being subjected to the molten wave of solder; the time-of-exposure to the molten solder must be long enough to heat the greatest thermal sink locality on the PWB, resulting in unnecessary thermal stressing of all other areas of the PWB and its associated components; and the exposure of the PWB to high temperatures requires special treatment to prevent moisture outgassing and associated delamination of the epoxy glass board material. Additionally, wave soldering may result in many defective or non-uniform joints. Therefore, in order to measure the actual efficiency of the wave-soldering method, consideration must be given to the time and effort required to mechanically fasten the components in position, the wave-soldering process itself, and the detection and repair of defective soldered joints. The main categories of defective joints made by wave soldering are: insufficient or excess solder, voids, no solder, or pinholes in the joint, and solder bridging between circuits, and to a lesser extent lifted solder pads. Discrete joint soldering, of course, is beneficial in that it permits optimization of the process parameters required for each joint on the PWB, resulting in high quality joints with minimum defects. However, to enable discrete joint soldering to be economically feasible when compared to wave soldering, certain problems must be overcome; particularly where the thermal energy required to heat the joint is conducted through the soldering tip or probe, such as a soldering iron. For example, the cross-section of the probe should be preferably approximately the same size as the diameter of the solder pad in order to restrict the heat applied exclusively to the target joint (typically 0.050") diameter. This small size imposes practical limits on the rate at which the thermal energy can be transferred to the joint by conduction. This in turn, particularly for joints with large heat sinks (i.e. buss planes), creates a significant time lag when the thermal energy stored in an idle probe is initially drawn into the joint at a higher rate than can be resupplied from a remote power (heat) source. This time/temperature relationship results in uncontrolled and unnecessary heating of the surrounding board laminate and the associated component. Further, for discrete-joint soldering, the probe material should on the one hand be a good thermal conductor such as most metals, and on the other hand not corrode or degrade in any way that would have an adverse effect on conduction of heat into the joint. Maintenance of a constant, and therefore predictable, conductance factor, appears to be a requisite for an automated, discrete joint soldering process.
In an attempt to overcome the disadvantages of discrete-joint soldering when performed with a soldering iron, it has been proposed to utilize a laser beam as the energy source for surface-mounted components, where the solder must be applied to the same side of the PWB as the components are mounted. However, as far as is known, a method and apparatus for making discretely soldered joints in through-hole PWB's automatically, and thereby overcoming the disadvantages of wave soldering, has not been known prior to the present invention.